1. Field of the Invention
The present invention relates to a method for manufacturing solar cells, and more specifically thin-film solar cells.
2. Description of the Related Art
The technique used to produce thin-film solar cells is the PVD-technique (Physical Vapor Deposition), in which copper, indium and selenium are vaporized and deposited on a heated substrate. The substrate normally includes a glass plate on which a layer of molybdenum (Mo) is sputtered. The compound copper-indium-diselenide (CuInSe2) is formed on the substrate. A layer of cadmium sulphide (CdS) and a transparent electric contact in the form of doped zinc oxide (ZnO) are laid over a layer of cadmium sulphide (CdS).
Vaporization of the various substances takes place in a chamber. When the glass plate has large dimensions, the plate is positioned stationary in relation to the sources of the aforesaid substances.
Three problems are primarily encountered when producing thin-film solar cells based on CuInSe2 by means of vacuum deposition (PVD). This also applies with layer compositions other than just CuInSe2.
The first of these problems resides in the ability to produce layers on large surfaces in a manner which will provide a uniform layer. Thin-film solar cells normally have a size of 1.times.0.4 meters. The thickness of the CuInSe2-layer is particularly critical and should be uniform in order to achieve good functioning of the solar cell.
The second of the aforesaid problems is that the composition of the CuInSe2-layer should vary over its thickness, so that the composition of that part of the layer furthest away from the glass plate will contain more indium than is given by the stoichiometric composition. Correspondingly, that part of the layer which lies nearest the glass plate should contain a relatively higher percentage of copper. This is because of the desire to suppress the formation of so-called copper nodules on the upper boundary surface of the layer, since such nodules reduce the efficiency of the solar cell.
The known technique solves this problem by first applying a layer rich in copper and then applying to this first layer in a second process stage a second layer which is rich in indium. Application of a large quantity of indium suppresses the formation of a copper-rich surface, which in turn suppresses the formation of said nodules. This known technique thus requires two different process stages, thereby rendering the process complicated and expensive.
The third of the aforesaid problems resides in a relatively low yield of vaporized substances that are deposited on the substrate.
These problems are solved by means of the present invention.